1 00:00:00,000 --> 00:00:07,070 2 00:00:07,070 --> 00:00:07,710 PROFESSOR: Hi. 3 00:00:07,710 --> 00:00:09,900 In this clip we're going to demonstrate the gene 4 00:00:09,900 --> 00:00:12,460 regulation of the E. coli lac operon. 5 00:00:12,460 --> 00:00:15,260 As you remember from the lecture clips, the lac operon 6 00:00:15,260 --> 00:00:18,120 is a collection of genes that produces the lactose 7 00:00:18,120 --> 00:00:20,990 metabolism machinery when expressed. 8 00:00:20,990 --> 00:00:24,080 Now, when these are expressed, E. coli is able 9 00:00:24,080 --> 00:00:25,660 to metabolize lactose. 10 00:00:25,660 --> 00:00:29,070 However, it prefers to metabolize glucose, to use 11 00:00:29,070 --> 00:00:30,510 that as its sugar. 12 00:00:30,510 --> 00:00:34,710 Thus, when there's glucose available to the cell, it's 13 00:00:34,710 --> 00:00:39,480 pointless for it to create this machinery, because it's 14 00:00:39,480 --> 00:00:41,000 not going to actually use it. 15 00:00:41,000 --> 00:00:43,780 It's going to metabolize the glucose instead. 16 00:00:43,780 --> 00:00:47,310 But when we remove glucose and add lactose, the E. coli 17 00:00:47,310 --> 00:00:50,890 absolutely needs to be able to metabolize this lactose, 18 00:00:50,890 --> 00:00:53,080 otherwise it's not going to be able to grow with the 19 00:00:53,080 --> 00:00:55,280 available energy source. 20 00:00:55,280 --> 00:00:59,370 Thus, we have a couple of different situations. 21 00:00:59,370 --> 00:01:04,650 When we have glucose present but not lactose, we do not 22 00:01:04,650 --> 00:01:08,950 want to express the lac operon. 23 00:01:08,950 --> 00:01:13,690 However, when we remove glucose and we add lactose, we 24 00:01:13,690 --> 00:01:18,620 do want to express the lac operon. 25 00:01:18,620 --> 00:01:23,250 Now we've covered when the E. coli would like to express 26 00:01:23,250 --> 00:01:24,300 this operon. 27 00:01:24,300 --> 00:01:28,600 However, how can it express this operon only when it wants 28 00:01:28,600 --> 00:01:30,610 to and not when it doesn't? 29 00:01:30,610 --> 00:01:34,000 That's where the promoter comes into play. 30 00:01:34,000 --> 00:01:37,350 The promoter is where RNA polymerase binds. 31 00:01:37,350 --> 00:01:40,440 And RNA polymerase needs to bind to this promoter in order 32 00:01:40,440 --> 00:01:44,850 to transcribe and express these genes. 33 00:01:44,850 --> 00:01:48,550 In this particular promoter, we have a site where a 34 00:01:48,550 --> 00:01:52,480 repressor can bind and a site where an activator can bind. 35 00:01:52,480 --> 00:01:58,810 So I have a repressor and I have an activator. 36 00:01:58,810 --> 00:02:02,130 37 00:02:02,130 --> 00:02:07,920 So when the repressor is bound, polymerase cannot bind 38 00:02:07,920 --> 00:02:09,650 to the promoter. 39 00:02:09,650 --> 00:02:13,240 Polymerase also needs the activator to be bound to the 40 00:02:13,240 --> 00:02:15,640 promoter in order to bind. 41 00:02:15,640 --> 00:02:18,910 So how can the E. coli regulate when the repressor 42 00:02:18,910 --> 00:02:21,710 and activator are bound to the promoter? 43 00:02:21,710 --> 00:02:27,210 Well, when we have lactose in the system, I have lactose 44 00:02:27,210 --> 00:02:31,510 right here, the repressor is able to bind lactose. 45 00:02:31,510 --> 00:02:33,830 The repressor only binds to the promoter 46 00:02:33,830 --> 00:02:35,240 when there is no lactose. 47 00:02:35,240 --> 00:02:40,130 And when binding to lactose, the repressor comes undone 48 00:02:40,130 --> 00:02:41,380 from the promoter. 49 00:02:41,380 --> 00:02:43,240 50 00:02:43,240 --> 00:02:45,970 How does the cell tell whether there's glucose 51 00:02:45,970 --> 00:02:47,760 in the cell or not? 52 00:02:47,760 --> 00:02:51,210 When glucose is low in the cell, the cell makes a 53 00:02:51,210 --> 00:02:53,990 molecule called cyclic AMP, or CAMP. 54 00:02:53,990 --> 00:02:57,160 I have a molecule of CAMP right here. 55 00:02:57,160 --> 00:03:00,210 And so CAMP is the cell's way of indicating that there is 56 00:03:00,210 --> 00:03:02,700 low glucose in the cell. 57 00:03:02,700 --> 00:03:05,640 The activator needs CAMP to bind to be 58 00:03:05,640 --> 00:03:07,940 activator binding site. 59 00:03:07,940 --> 00:03:11,730 When CAMP is removed, or glucose is added, the 60 00:03:11,730 --> 00:03:16,440 activator cannot bind to the promoter anymore. 61 00:03:16,440 --> 00:03:19,240 Ok, so let's give it a try. 62 00:03:19,240 --> 00:03:24,010 In this first situation we have glucose, 63 00:03:24,010 --> 00:03:26,000 we don't have lactose. 64 00:03:26,000 --> 00:03:31,415 So I have neither my lactose nor my CAMP in the system. 65 00:03:31,415 --> 00:03:36,440 66 00:03:36,440 --> 00:03:38,390 Repressor, can you bind? 67 00:03:38,390 --> 00:03:39,650 The Repressor is bound. 68 00:03:39,650 --> 00:03:41,140 Activator, can you bind? 69 00:03:41,140 --> 00:03:43,500 No, the activator is not bound. 70 00:03:43,500 --> 00:03:48,180 The repressor is bound, and so when RNA polymerase comes to 71 00:03:48,180 --> 00:03:51,490 the promoter, RNA polymerase is not able to 72 00:03:51,490 --> 00:03:53,930 bind at this promoter. 73 00:03:53,930 --> 00:03:57,140 And thus, it's not able to express these genes. 74 00:03:57,140 --> 00:03:59,800 75 00:03:59,800 --> 00:04:04,850 And so, as we can see here, there is no expression. 76 00:04:04,850 --> 00:04:08,740 In this second situation, we have no glucose 77 00:04:08,740 --> 00:04:11,770 and we do have lactose. 78 00:04:11,770 --> 00:04:17,180 Since I'm removing glucose, I am adding back CAMP. 79 00:04:17,180 --> 00:04:18,950 Activator, can you bind? 80 00:04:18,950 --> 00:04:20,730 Activator is now bound. 81 00:04:20,730 --> 00:04:24,145 And I'm also adding lactose to the system. 82 00:04:24,145 --> 00:04:26,820 83 00:04:26,820 --> 00:04:29,790 Repressor can't bind anymore. 84 00:04:29,790 --> 00:04:31,820 So what do I have at my promoter now? 85 00:04:31,820 --> 00:04:34,480 RNA polymerase comes along and finds only an 86 00:04:34,480 --> 00:04:36,150 activator at the promoter. 87 00:04:36,150 --> 00:04:40,080 The polymerase can thus bind to the promoter and move 88 00:04:40,080 --> 00:04:44,770 downstream to transcribe and express the lactose metabolism 89 00:04:44,770 --> 00:04:46,270 machinery that we have here. 90 00:04:46,270 --> 00:04:53,110 91 00:04:53,110 --> 00:04:59,400 So we have now covered how the E. coli organism is able to 92 00:04:59,400 --> 00:05:03,290 regulate the transcription of the 93 00:05:03,290 --> 00:05:05,590 lactose metabolism machinery. 94 00:05:05,590 --> 00:05:09,710 This is only one example of many genes and many organisms 95 00:05:09,710 --> 00:05:13,050 that are regulated by mechanisms such as this. 96 00:05:13,050 --> 00:05:16,320 Gene regulation is a very widespread mechanism, in order 97 00:05:16,320 --> 00:05:18,950 to keep a cell from producing proteins that 98 00:05:18,950 --> 00:05:20,820 are a waste to produce. 99 00:05:20,820 --> 00:05:22,070 Thank you for watching. 100 00:05:22,070 --> 00:05:23,712